This invention relates to stability additives for perfluoropolyalkylether fluids.
Highly fluorinated compounds have long been of interest because of their excellent potential for high temperature applications. Fluids based on perfluoropolyalkylethers (PFPAE) have, in addition to high thermal and oxidative stability, a wide liquid range which make them ideal candidates for aerospace applications. These fluids consist essentially of a mixture of fluorinated polyethers. These fluids have the general formula: EQU R.sub.f O(Z).sub.m (Y).sub.n R.sub.f
wherein R.sub.f is a lower perfluoroalkyl group, such as CF.sub.3, C.sub.2 F.sub.5, C.sub.3 F.sub.7 and the like, wherein Z is --CX.sub.2 CX.sub.2 O--, --CX.sub.2 CX.sub.2 CX.sub.2 O-- or --CX.sub.2 OCX.sub.2 CX.sub.2 O--, where X is --F, --CF.sub.3, --C.sub.2 F.sub.5 and the like, and Y is --CFXO--, m and n are integers whose sum is between 2 and 200 and the ratio of n to m is between 0.1 and 10, and wherein the Z and Y units are statistically distributed along the PFPAE chain. Commercial base fluids of this type have been available for some time, for example, Krytox.RTM. (DuPont), Fomblin.RTM. (Ausimont), Demnum.RTM. (Daikin) and the like. Their practical utility in aerospace and military applications has been hampered by the wear and corrosion of certain metal components exposed to these base fluids under extreme conditions.
Deficiencies in base fluids are generally removed and the performance of the fluids improved by the use of additives. Conventional additives developed for the improvement of a variety of specific properties of hydrocarbon base fluids are generally not suitable for perfluorinated fluids. These conventional additives are not soluble in perfluorinated fluids and are ineffective. One way of overcoming this incompatibility is to synthesize compounds containing fluoroalkylether groups plus selected functional groups for specific activity. Although this approach may make the compound soluble in a fluorinated base fluid, mere replacement of hydrocarbon groups with fluorocarbon groups can change the useful properties of the additive itself by changing the properties of the critical functional group present in the additive. These difficulties are well known to those familiar with the art. In spite of these difficulties, a few useful additives have been developed for perfluorinated fluids. One such example is the development of PFAE substituted triphenylphosphines, C. E. Snyder, Jr. and C. Tamborski, U.S. Pat. No. 4,097,388. These additives, when dissolved in PFPAE fluids, have significantly reduced the corrosion of certain metal components exposed to the fluid at high temperatures in an oxidative environment.
We have discovered a group of compounds which do not contain fluoroalkylether groups, yet are soluble in fluorinated base fluids. These compounds enhance the stability of fluorinated base fluids.
Accordingly, it is an object of this invention to provide novel stability enhancing additives for perfluoropolyalkylethers.
Another object of this invention is to provide perfluorinated fluids having improved stability properties.
Other objects and advantages of the invention will be apparent to those skilled in the art.